Light-emitting semiconductor devices, such as laser diodes, laser diode arrays, and light emitting diodes (LEDs), are frequently used in conjunction with optical fibers for delivering emitted light to external objects. The efficiency of optical coupling between a semiconductor chip and an optical delivery fiber must be maintained during normal operation of the fiber coupled semiconductor device.
A light emitting area of most semiconductor chips is quite small, measuring only a few microns in a direction perpendicular to a plane of thin film layers of the semiconductor device. Generally, small size of the light emitting area is a good, desirable property of a light source, because it is associated with high brightness of the source, allowing the light from the source to be tightly focused, for example. To preserve the brightness of the semiconductor source, it is preferable to use optical fibers having small core diameter. Due to small dimensions of the light emitting area and the small fiber core diameter, the optical fiber has to be precisely aligned to the semiconductor chip. Furthermore, to maintain the emitted power level over a lifetime of the device, the precise alignment between the optical fiber and the semiconductor chip must be maintained over lifetime of the device.
Referring to FIG. 1, a prior-art fiber-coupled laser diode assembly 10 is shown. The laser diode assembly 10 has been disclosed by Ziari et al. in U.S. Pat. No. 6,758,610 assigned to JDS Uniphase Corporation and incorporated herein by reference. The laser diode assembly 10 includes a base 11, a laser chip submount 12, a laser chip 13, a fiber sub-mount 14 including a top section 14A, and an optical fiber 15. The submounts 12 and 14 are affixed to the base 11 with solder layers 16, and the laser chip 13 is affixed to the laser chip submount 12 with the solder layer 16. A solder bead 17 is used to connect the optical fiber 15 to the fiber submount 14. The fiber 15 is metalized to have a metallization layer 18 for improving wettability of the optical fiber 15 by the metal solder material of the bead 17. The top section 14A has low thermal conductivity to serve as a thermal barrier during soldering operation. A front surface 19 of the optical fiber 15 is lensed to improve fiber coupling efficiency.
Disadvantageously, the fiber coupling efficiency of the laser diode assembly 10 can vary with ambient temperature even in cases when the laser diode assembly 10 is temperature stabilized using a thermoelectric cooler (TEC). The TEC is not shown in FIG. 1. To remove the heat from the laser diode assembly 10, the base 11 is attached to a top surface of the TEC, and a bottom surface of the TEC is connected to an external heat sink, not shown. When the ambient temperature is different from the temperature of the base 11 of the laser diode assembly 10, the inside and the outside surfaces of the TEC are at different temperatures. The temperature difference results in deformation of the inside TEC surface on which the base 11 is mounted, which causes the base 11 of the laser diode assembly 10 to deform. The deformation of the base 11 results in misalignment of the optical fiber 15 relative to the laser diode chip 13, which results in a loss of optical fiber coupling efficiency, ultimately leading to a reduction of output optical power and the conversion efficiency of the laser diode assembly 10.
One prior-art approach to improving stability of fiber coupling efficiency in a fiber coupled diode laser is disclosed by Miyokawa et al. in U.S. Pat. Nos. 6,734,517 and 7,030,422, incorporated herein by reference. Miyokawa et al. disclose a semiconductor laser diode module constructed so as to reduce the temperature dependence of the fiber coupling efficiency. In the semiconductor laser diode module of Miyokawa et al., the material of a base for supporting a laser chip is selected to match that of a fiber holder. Further, the fiber holder has two parts, one of which is mounted to the base and the other supports the fiber ferrule. The part that is mounted to the base is shaped so that it does not interfere with the laser diode mounting region of the base. Disadvantageously, the module of Miyokawa et al. is rather complex, requiring many laser welding spots to affix all the parts of all the holder elements.
It is a goal of the present invention to provide a simple and inexpensive fiber coupled semiconductor device, in which the fiber coupling efficiency is maintained during normal operation at a varying ambient temperature. It is also a goal of the invention to provide a method of assembly of such a device.